Heart sounds contain a great deal of diagnostic information. A stethoscope is a standard instrument used in clinical examinations and has aided in enabling clinicians to become familiar with normal and abnormal heart and lung sounds. Clinicians typically refer to four heart sounds, S1, S2, S3 and S4. The first heart sound, S1, corresponds to the start of ventricular systole and is generated by the abrupt closure of the mitral and tricuspid valves between the ventricles and atria as ventricular pressure exceeds atrial pressure. The second heart sound, S2, is generated by the closure of the aortic and pulmonary valves, near the end of ventricular systole and start of ventricular diastole.
The third heart sound, S3, is associated with early, passive diastolic filling of the ventricles, and the fourth heart sound, S4, is associated with late, active filling of the ventricles due to atrial contraction. The third sound is generally difficult to hear in a normal patient using a stethoscope, and the fourth sound is generally not heard in a normal patient. Presence of the third and fourth heart sounds may indicate a pathological condition.
Implementation of acoustical sensors in implantable medical devices such as cardiac pacemakers has been proposed because of the valuable information that can be obtained relating to the occurrence of the standard heart sounds S1, S2, S3 and S4 (if present). In addition to the standard S1 through S4 heart sounds, a large amount of acoustical information correlated to blood flow through the heart chambers, valves and arteries could potentially be gleaned from an implantable acoustical sensor. However, even when implanted, an acoustical sensor positioned to sense cardiac sounds will be subjected to lung and other physiological sounds and non-physiological noise. For example, heart sound sensing using a piezoelectric transducer positioned in the pacemaker housing will be somewhat hampered due to filtering of high frequency cardiac sounds by the lungs and the influence of respiration on the acoustical signal. A need remains, therefore, for an implantable acoustical sensor useful for monitoring cardiac sounds with high specificity and sensitivity.